1. Field of the Invention
The present invention relates to a method of manufacturing an integrated thin-film solar battery by forming a plurality of unit elements connected to each other on a substrate and an integrated thin-film solar battery manufactured by that method, and more particularly to an improvement in the electrical characteristics of the solar battery by improving the quality of an interface between a semiconductor layer and an electrode layer at a side opposite the light receiving surface, thereby realizing an integrated thin-film solar battery with a high output.
2. Description of the Related Art
The use of solar batteries that convert energy of solar light directly into electrical energy has been now started on a large scale, and crystal-based solar batteries exemplified by single-crystal silicon, polycrystal silicon or the like have already been put into practical use as an outdoor power solar battery. On the other hand, attention has been drawn to a thin-film solar battery such as of amorphous silicon as an inexpensive solar battery because the cost of raw materials used therefor is reduced. However, in general, such a thin-film solar battery is still being developed, and extensive research and development have been conducted in order to develop the thin-film solar battery into a solar battery used outdoors on the basis of the experience of using the thin-film solar battery as a power supply for consumer electronic devices such as calculators.
The thin-film solar battery, as in conventional thin film devices, is manufactured by a process in which the deposition of thin films using CVD sputtering or the like, and patterning of the thin films are repeated so as to obtain a desired structure. There is normally adopted an integrated structure in which a plurality of unit elements are connected in series or parallel on a single substrate. The power solar battery for use outdoors requires a large area substrate that exceeds, for example, 400.times.800 (mm).
FIG. 2 is a cross-sectional view showing the structure of the above-mentioned conventional thin-film solar battery. This structure of an integrated thin-film solar battery has been generally used up to now, in which each unit element 15 has a first electrode layer 5, a semiconductor layer 9 made of an amorphous silicon or the like, and a second electrode layer 13, which are stacked one on another in the stated order. The unit elements 15 adjacent to each other are connected in series through a connection opening 7 formed in the semiconductor layer 9. The first electrode layer 5 is usually formed of a transparent electrically conductive film made of, for example, tin oxide (SnO.sub.2), zinc oxide ZnO), or indium tin oxide (ITO), and the second electrode layer 13 is formed of a metal film made of, for example, aluminum (Al), silver (Ag), or chromium (Cr).
The above-mentioned conventional integrated thin-film solar battery is manufactured by a method which will be described hereinafter with reference to FIG. 2. On a glass substrate 3, the transparent electrically conductive film made of SnO.sub.2, ZnO, or ITO is deposited as the first electrode layer 5, and the first electrode layer 5 is divided into a plurality of sections corresponding to the power generation regions by laser-scribing. Then, a cleaning of the first electrode layer 5 is conducted in order to remove the debris melted and cut off by the laser-scribing. A semiconductor layer 9 made of amorphous silicon with a p-i-n junction structure is deposited on the overall surface of the first electrode layer 5 through a plasma CVD technique. Subsequently, as with the first electrode layer 5, after the semiconductor layer 9 has been divided into a plurality of sections through the laser scribing technique, the semiconductor layer 9 is cleaned in order to remove the debris melted and cut off by the laser-scribing. Each of the plurality of semiconductor layer 9 sections are formed on top of the substrate 3 and the first electrode layer 5 so as to bridge at least two adjacent sections of the plurality of first electrode layer 5 sections. The connector opening 7 (e.g. a VIA hole) is etched in each of the semiconductor layer 9 sections in the vicinity of an adjacent first electrode layer 5 section bridged by the semiconductor layer 9.
Further, as the second electrode layer 13, a metal film made of Al, Ag, Cr or the like is deposited on the semiconductor layer 9 as a single layer or a plurality of layers, and divided into a plurality of sections 13 through the laser scribing technique as with the first electrode layer 5. The second electrode layer 13 contacts the first electrode layer 5 through the connector opening 7 in each of the semiconductor layer 9 sections when filled in with the second electrode layer 13 material, thus completing an integrated thin-film solar battery having a large area.
However, in the above-mentioned conventional integrated thin-film solar battery, it is found that the fill factor (FF value) of its output characteristics is low. Generally, in the manufacture of the integrated thin-film solar battery, the individual cell parameters such as the thickness of the respective electrode layers 5 and 13 or the quality of the semiconductor layer 9 are optimized in order to improve the characteristics. Since the large area of the substrate 3 makes experiments for optimizing the individual process conditions complicated, experimental thin-film solar batteries having a small area are normally manufactured through an easy process as preceding experiments, for evaluating the characteristics obtained thereby. Then, the optimum conditions of the individual processes obtained by the above manner are incorporated the process of manufacturing a thin-film solar battery having a large area.
However, although excellent numerical values may be obtained by the preceding experiments, when the optimum conditions thereof are incorporated in the process of manufacturing a thin-film solar battery having a large area, excellent results as good as the numerical values obtained by the preceding experiment cannot be obtained, and most of the results are lower in FF value. Hence, an improvement in the above-mentioned FF value is indispensable and urgently required for integrated thin-film solar batteries having a large area, in order to improve the conversion efficiency.
Under these circumstances, the present inventors have studied in detail the cause of the lowering of the FF value, with the result that they have proved that the interface between the semiconductor layer 9 and the second electrode layer 13 causes the FF value to be lowered.
FIG. 3 shows a cross-sectional structure of a thin-film solar battery having a small area, used in the above-mentioned preceding experiments. The thin-film solar battery having a small area is obtained in such a manner that a first electrode layer 5 made of SnO.sub.2, ZnO, ITO or the like, a semiconductor layer 9 made of an amorphous silicon or the like, and a second electrode layer 13 made of Al, Ag, Cr or the like are stacked on a substrate 3 in the stated order, and patterning of the periphery of the second electrode layer 13 and the semiconductor layer 9 is conducted. The characteristics of the solar battery are then measured by placing a measuring probe on an exposed portion 5a of the first electrode layer 5 and the second electrode layer 13. The thin film solar battery having a small area is not subjected to a cleaning processing since the semiconductor layer 9 is deposited on the substrate 3 before the second electrode layer 13 is deposited thereon, the first electrode layer 5, the semiconductor layer 9, and the second electrode layer 13 being continuously successively formed. In other words, it has been proved that the interface between the semiconductor layer 9 and the second electrode layer 13 absorbs moisture, etc., and the generation of a natural oxide film on the amorphous silicon surface occurs, with the result that the FF value is lowered. However, in the case of the integrated thin-film solar battery having a large area, because the laser scribing technique is applied for patterning, debris melted and cut off by the laser-scribing unavoidably occur. Unless the debris are removed, the adhesion of the second electrode layer 13 and the first electrode layer 5 in the connecting opening 7 is degraded. This adversely affects the characteristics and reliability of the solar battery more than the lowering of the FF value. Hence, the cleaning processing for removing the debris melted and cut off by laser-scribing is essential in the process of manufacturing the integrated thin-film solar battery having a large area.
The above-mentioned conventional method makes it impossible to improve the FF value while conducting the cleaning processing.